The present invention relates to calibration systems, to methods for producing such systems and to valve elements useful in such systems. More particularly, the invention relates to calibration systems for sensors, such as blood sensors, to methods for producing such systems so as to provide for effective sensor calibration and performance even after long term storage and to valve elements which are important components of such systems.
It is often necessary or desirable to monitor various parameters of blood and to obtain quantitative data concerning such parameters in real time. In order to accomplish this, blood is caused to flow through a flow-through housing past sensors which provide signals representative of the parameters of interest. For example, Cooper U.S. Pat. No. 4,640,820 shows a flow-through housing with fluorescent sensors which respond to the partial pressure of oxygen, the partial pressure of carbon dioxide and the pH of blood which has passed through the flow-through housing.
Prior to using the flow-through housing, the sensors must be calibrated. One calibration technique, which is used for the sensors of the Cooper patent, is to attach the sensor carrier to a calibration housing containing calibration liquid. This places the sensors in communication with a relatively large cross-sectional area passage. The gas or gases of interest are then bubbled through the calibration liquid. A similar technique is utilized to calibrate the sensors shown in Maxwell U.S. Pat. No. 4,830,013.
For some applications, it is desirable to utilize a flow-through housing having a relatively small cross-sectional area and to maintain that area sterile during calibration. In fact, the cross-section area is sufficiently small so that, when gas is passed through the liquid, the surface tension may cause the calibration liquid to be expelled from the passage and prevent exposure of the sensors to the gases in the calibration liquid and thus cause an inaccurate calibration.
Maxwell et al U.S. Pat. No. 5,057,278 discloses a very useful sensor calibration system for sensors associated with flow-through cassettes. The disclosure of this patent is incorporated in its entirety herein by reference.
It is beneficial that sensors in such flow-through cassettes be maintained wet with calibration liquid during storage, even during storage for periods of months or even up to about one year. Since it is not desirable to fill the entire flow-through or endless loop of the calibration system with calibration liquid, for example, because such large amount of calibration liquid can unduly increase the amount of time required to effectively calibrate the sensors, the calibration system should be manufactured and maintained so that a limited amount of the calibration liquid is used and effectively wets the sensors.
Prior art systems have effectively maintained the sensors wet with calibration liquid by stopping the flow of calibration liquid through the endless loop. This has been done, for example, by providing an elongate section of tubing in the endless loop. The elongate section of tubing is sharply kinked during packaging and a retainer or clamp is secured to the tubing to maintain the tubing completely occluded. Further, the interior of the tubing is coated with a material, such as a polyxylylene, which prevents the kink in the tubing from becoming permanent. When the calibration system is ready for use, the clamp is removed from the tubing and the tubing is unkinked, thus allowing calibration liquid to be pumped through the endless loop.
Although the kinked tube and clamp effectively provide that the sensors remain wet with calibration liquid during storage, this approach does have certain disadvantages. For example, the long length of tubing adds to the volume of the endless loop and is an additional component of the calibration system which must be assembled. In addition, the retainer or clamp must be removed before use, which removal is an additional step and may require a special removal tool. Further, there is some chance of damaging the tubing in removing the retainer or clamp, which is fastened directly on the tubing.
It would be advantageous to provide a new calibration system for maintaining such flow-through cassette sensors wet with calibration liquid during storage.